Combustion chamber for axial flow gas turbines



April 1956 M. s. KUHRING 2,742,762

COMBUSTION CHAMBER FOR AXIAL FLOW GAS TURBINES Filed May 31, 1951 Unite States PatentO COMBUSTION CHAMBER FOR AXIAL FLOW GAS TURBINES Application May 31, 1951, Serial No. 229,071

2 Claims. (Cl. 6039.65)

This invention relates to combustion chambers for gas turbines and the like and more particularly it relates to a means for overcoming the problems created by the formation of carbon in the primary combustion zone, which carbon formation under normal circumstances tends to block the primary air inlets, necessitating dismantling and cleaning of these ports to restore efficient operation of the combustion chamber.

Combustion chambers for gas turbines and the like are generally constructed from very thin sheet metal in order to overcome a situation known as thermal shock which arises due to the material being exposed to extreme ranges of temperature. main uses in which gas turbines have heretofore been employed is in jet propulsion units for aircraft, it is desirable in most instances to keep the overall weight of all component parts of gas turbine combustion chambers as low as is possible, consistent with the parts having the required strength to fulfill the function that they are to perform.

Gas turbine combustion chambers which are in use in aircraft jet propulsion units may be divided into two general types. In one type the combustion chamber is generally cylindrical in form and has an inner wall or flame tube defining the combustion chamber proper which is closed at the forward end, except for the primary air holes which are described below, and an outer wall surrounding the inner wall through which air is supplied. Air is supplied to the inside of the combustion chamber proper through holes in the inner wall which are usually divided into two groups, the first of which is a group of holes in the forward end ofthe casing through which primary air is admitted, and the second group being rather larger holes spaced around theinner wall somewhat downstream of the forward end of the combustion chamber, for the admission of secondary combustion air. The fuelis normally supplied to this type. of combustion chamber by nozzle means extending into the interior of the combustion chamber proper, centrally at the forward end thereof and facing axially to the rear.

The second general type of combustion chamber is annular in form and differs from the first type above mentioned principally in this respect only. The essential parts and their position correspond to those detailed above for the cylindrical type of combustion chamber.

In both types of combustion chamber, the primary air and fuel become intimately mixed in proportions sulficient to support what is generally referred to as primary combustion." 'In the secondary combustion zone air is admitted to complete combustion. In practice these zones are not accurately defined but experience has shown that the most intense heat within the combustion chamber is produced within the, secondary combustion zone where the temperature of the combustion gases may reach figures of the order of 1750 C. On theother hand, the temperatures within the primary combustion .zone during operation are of a much lower order and the In addition, since one of the v A 2,742,762 Patented Apr. 24, 1956 temperature of the wall of the combustion chamber proper in the area of the primary air inlets is usually not; higher than about 750 C.

Since the maximum temperature of a surface upon which carbon mayform in an oxidizing atmosphere without being immediately burnt off is from about 660 to 750 C., depending to some extent upon the nature of the surface and the form of carbon which is deposited, it will be appreciated that whereas substantially no can bon will tend to build up on the walls of the combustion chamber in the secondary combustion zone because of the high temperatures existing there, the temperature of the wall of the combustion chamber in the region of the primary air inlets is such that carbon may form and build up' thereon, and it has been found in practice that considerable build up of carbon occurs around and about the inside end of the primary air inlet ports, which build up, after a number of hours of continuous operation of the combustion chamber, seriously restricts the flow of primary air into the combustion chamber and accordingly materially reduces the efficiency of the combustion chamber as a whole.

; Since it is not practical to raise the temperature of the wall of the combustion chamber in this area to a temperature above that which will permit the build up of carbon ice thereon, it has not been possible up to the present time, p

to prevent the building up of carbon in the primary air inlet ports and accordingly the length of time required for a build up of carbon in this area suflicient to materially reduce the efliciency of the combustion chamber has, up until the present time, been a limiting factor in the length of time for which jet propulsion units could be continuously operated without dismantling for maintenance purposes.

I have found that it is possible to control the deposit of carbon upon the interior wall of the combustion chamher in the area of the primary combustion air inlets in such a way that the build up of carbon does not restrict the primary air inlets so that the build up of carbon in this area is no longer so serious a limiting factor in the length of time for which a combustion chamber of this type may be continuously operated without serious loss of efliciency.

Briefly stated, my invention consists in forming the primary air inlets themselves in such a manner that the air stream flowing through them will completely fill the space provided at the extreme inner edge of the inlet port. This may be done in a number of ways without materially increasing the Weight of the combustion chamber or increasing its susceptibility to thermal shock but, generally speaking, all methods of doing this involve the provision of a somewhat elongated port which is sufliciently long that the air stream flowing through it under conditions of operation will fill the passage at its discharge end and said, that the ports may not be elongated by thickening an appreciable area of the wall'because of considerations of weight and thermal shock.

My invention and the theory upon which it operates, will be more fully described in the following detailed specification in conjunction with the accompanying drawmgs.

In the drawings:

Fig. 1 is a fragmentary cross section of a typical combustion chamber of the cylindrical type;

Fig. 2 is an enlarged fragmentary cross sectional view of the end of the combustion chamber showing the type of holes normally employed as primary'air inlet ports;

Fig. 3 is a greatly enlarged fragmentary view of one of the air inlet ports shown in Figure 2, illustrating the flow of air therethrough;

Fig. 4 is a greatly enlarged fragmentary cross section of one of the primary inlet holes illustrating the progressive build up of carbon which occurs during operation;

Fig. 5A is a fragmentary cross section of a primary air inlet port according to the invention showing the manner in which the carbon builds up in stages during operation without interfering with the flow of air therethrough;

Fig. SB is a similar fragmentary cross section illustrating an alternative form of the invention;

Fig. 5C is a similar fragmentary cross section illus- (rating a still further embodiment of the invention;

Fig. 5D is a fragmentary cross section of a still further embodiment of the invention illustrating the desired air flow characteristics according to the invention;

Fig. SE is a fragmentary cross section of a still further embodiment of the invention.

Fig. 6 is a front end elevation of an example of a primary air inlet port; according to the invention with typical dimensions; and

Fig. 7 is a longitudinal section of the air inlet port shown in Figure 6, taken along the line 7--7.

In general, a combustion chamber for internal cornbustion gas turbine engines, as illustrated in Figure 1, consists of an outer shell 7, an inner shell or flame tube 6, some form of fuel supply 2 and spray nozzle 1.

Air enters through the passage 3. The primary air passes through a plurality of holes 4 in the end or head of the inner shell or flame tube. These holes are intentionally small to permit the flow only of sufficient air to support combustion in this area of the flame tube. This air flow is shown by the dotted arrows and is known as primary air.

The greater part of the air flows around the head or closed end of the flame tube and passes through the annular space 8 between the walls 6 and 7, entering the secondary and tertiary air holes 5.

Because there is a moderate resistance to the flow through holes 4, and little resistance to flow through holes 5, the secondary air flow follows generally the pattern shown by the solid arrows. This promotes a considerable amount of turbulence in the primary zone and fuel droplets and vapor are intimately dispersed in the air.

The flame tube 6, including the head, normally operates at somewhat high temperatures. However, the closed head or end 9 is not nearly as hot as the open end and fuel droplets or vapor impinging on the surface of the head tend to form carbon which, in a period of time, tends to close or partially close the primary air holes 4 blocking the flow of primary air. This may limit the period which may be declared as the service life of the cornbustion chambers as the balance of air flow through the various portions of the combustion chamber are vitally important.

For reasons of lightness (for aircraft engines) and to prevent excessive thermal distortion or thermal shock (in all installations) it is desirable to keep the thickness of the flame tube to a minimum, consistent with strength.

The primary holes 4 are usually punched or drilled and appear as shown in Figure 2.

Air flow through a simple hole of this nature in a thin plate produces what is known as a vena contracta or contraction of flow. Increasing the thickness of the plate and, consequently the length of the passage, alleviates this condition somewhat but not entirely and in addition increases the weight and imposes greater thermal loads on the material.

The vena contracta induces an annular zone of turbulent back flow on the downstream side of the plate, as shown by the arrows in Figure 3. This air flow causes fuel in one form or another to impinge on the surface of the plate in and around the holes and carbon is deposited progressively, as shown at a, b and c in Figure 4. In severe conditions, the air flow is completely shut off in this area and not only has the efficiency of the combustion chamber been impaired progressively, wasting fuel, rendering the engine harder to start and raising turbine temperatures, but it becomes necessary to dismantle the combustion chambers and remove the carbon.

According to my invention the plate is effectively thickened locally, and the passage is improved aerodynamically so that while the original flow rate will be maintained, with the improved flow pattern and elimination of local eddies and fuel impingement at the edges of the primary air holes, carbon build up does not block the holes, but'takes place substantially as shown at a b and c in Figures 5A, 5B and 5C.

The thickening of the plate locally may be accomplished by an actual integral thickening of the plate as at 10, shown in Figure 5B, or by the fitting of a separate piece or liner IL in the form of a hollow rivet 12, preferably of thin metal, to the drilled or stamped hole, as shown in Figure 513 or by the insertion of a machined or formed hollow metal liner or passage as shown in Figure 5C. In all instances the resulting passage should be longer than the original thickness of the plate in which it is fitted, and it may be of any length desired, although there is no advantage in increasing its length beyond three to four times its internal diameter. The critical feature is that under conditions of operation there must be full flow at the extreme inside end of the passage, as shown at 13 in Figure 5D.

The insert must be fixed rigidly to the plate so that it will not become detached either by the flow of gases, from vibration, dirierential thermal expansion or other cause. For this reason, it may be necessary for the insert to project slightly downstream from the plate. This length should be kept to a minimum to assist in'preventing the insert from being heated by the direct action of the burning gases in the combustion chamber. It has been shown that if the insert does project materially inside the combustion chamber, carbon tends to build up on the outer surface of the insert and, while it does not tend to block the hole, this is not desirable. In Figures 5D and C are shown alternative methods by which the hollow rivet 12 may be attached without projecting downstream beyond the surface of the plate.

The upstream end of the insert should be in the form of what is known as a bellmouth. However, any similar form, such as an enlargement of the entry, countersinking, etc. will improve the air flow pattern.

When the insert is used, the internal diameter may be reduced slightly if the same rate of mass air fiow is desired or the number of holes may be reduced to maintain the same rate of mass flow, due to the higher coeflicient of discharge of such an orifice compared to that of a straight punched or drilled hole.

The following example illustrates one form of the in vention and provides dimensional data in respect to a type of primary air inlet-port according to the invention which was used in an experimental combustion chamber.

EXAMPLE 1 The end of a flame tube of a combustion chamber was punched with a number of holes in the normal way, as illustrated in Figure 2. In a number of the holes thus punched were secured the inserts shown in Figures 6 and'7. These inserts were secured on the upstream face of the end of the flame tube, and the actualdimensions of the air inlet port insert were as follows:

A inches .130 to .120 B dn .062 to .058 R "do-.." 0.156 Angle C 14 D ..mches .187 to .180 E o-.. .390 to .380 F do .248 to .246 G do .1880 to .1876 H do.. .125 to .124

vin Figure 4, while the carbon build up around the ports provided with the insert according to the invention built up in the manner illustrated in Figures 5A, 5B and 5C. At the conclusion of the test the airinlet ports without the inserts were substantially blocked off with carbon formation, while those air inlet ports having the inserts remained substantially free of obstruction by carbon formation.

It will be appreciated from the above that my invention provides a very simple means of controlling the build up of carbon around the primary air inlet passageways of a combustion chamber in such a way that the deposition of carbon does not have an adverse effect upon the etficiency of the primary air inlet ports.

What I claim as my invention is:

1. In an axial flow gas turbine in combination; a combustion chamber formed from thin sheet material and having a closed inlet end and an open discharge end; fuel inlet means adjacent said closed end; and a plurality of generally tubular inserts extending through said closed end in axial alignment with said combustion chamber and surrounding said fuel inlet means, said inserts each having a length which is substantially greater than the thickness of the material from which said end is formed, not extending appreciably into said combustion chamber, and being formed to provide an inlet passage for primary air which is arranged to project a stream of air axially into said combustion chamber which is of coincident cross section with the innermost extremity of said passage whereby build-up of carbon deposit around said'innermost extremity is confined to an annular space surrounding said airstream, and growth of said deposit does not tend to restrict the flow of said airstream.

2. The combination claimed in claim 1 in which, the passageway formed within said insert is formed with a bell mouth at its inlet end.

References Cited in the file of this patent UNITED STATES PATENTS 2,227,666 Noack Jan. 7, 1941 2,471,101 Feinberg May 24, 1949 2,510,645 McMahan June 6, 1950 2,547,619 Buckland Apr. 3, 1951 2,601,390 Hague June 24, 1952 2,631,429 Jacklin, Jr. Mar. 17, 1953 FOREIGN PATENTS 650,462 Great Britain Feb. 28, 1951 650,528 Great Britain Feb. 28, 1951 

